Principled BSDF image texture isn't appearing
You are plugging a shader node (the Principled BSDF node) into another shader (Diffuse BSDF node). This is not how Cycles' shaders work. You should remove the Diffuse BSDF and things will start working.

Blender tries to imply you shouldn't do this by colouring the node sockets differently. A green output (indicating a 'shader') shouldn't be plugged into a yellow input socket (colour data). However, a green shader output socket should be plugged into a green input socket.

You can read more about node socket types here:

What is the meaning of the color of the node sockets in the node editor?